Light-emitting device and manufacturing method thereof

ABSTRACT

To provide a light-emitting device using a nitride semiconductor which can attain high-power light emission by highly efficient light emission and a manufacturing method thereof, the light-emitting device includes a GaN substrate and a light-emitting layer including an InAlGaN quaternary alloy on a side of a first main surface of GaN substrate.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a Continuation of U.S. application Ser. No.10/916,802 filed Aug. 11, 2004, the entirety of which is incorporatedherein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a lighting device and a manufacturingmethod thereof More specifically, the present invention relates to alight-emitting device using a nitride semiconductor emitting ultravioletlight and a manufacturing method thereof

2. Description of the Background Art

A GaN-based compound semiconductor functions as a blue TED (LightEmitting Diode) or an ultraviolet LED because of its large band gap, andis often used as an excitation light source of a white LED. Suggestionssuch as the following have been made for improvement of performance ofthe GaN-based LED which emits ultraviolet light having a shortwavelength.

(d1) Using a SiC substrate and an InAlGaN layer as a light-emittinglayer, and adjusting a composition of In, for example, in the InAlGaNlayer to increase efficiency of light emission within an ultravioletregion of 360 nm or shorter wavelengths (Japanese Patent Laying-Open No.2001-237455).

(d2) Using as a light-emitting layer a single-layer quantum wellstructure formed with Al_(0.1)Ga_(0.9)N layer/Al_(0.4)Ga_(0.6)N layerformed on a GaN substrate to increase brightness (T. Nishida, H. Saito,N. Kobayashi; Appl. Phys. Lett., Vol. 79 (2001) 711).

The above-described ultraviolet light-emitting device, however, has lowlight emission efficiency, and the light emission efficiency decreasesbecause of heat production when a large current is passed for use inillumination. A reason for the low light emission efficiency of theaforementioned ultraviolet light-emitting device is its high dislocationdensity in the substrate and the light-emitting layer, which dislocationworks as a non-radiative center. In particular, when a sapphiresubstrate is used, it does not dissipate heat efficiently and there is astrong tendency of light emission efficiency not to increase linearly inproportion to an input, but to be saturated halfway.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a light-emitting deviceenabling highly efficient light emission and high-power light emissionand a manufacturing method thereof.

A light-emitting device according to the present invention includes alight-emitting layer including an InAlGaN quaternary alloy on a side ofa first main surface of a nitride semiconductor substrate.

According to the above-described construction, as the nitridesemiconductor substrate having a low dislocation density is used, adensity of threading dislocations operating as non-radiative centers inthe light-emitting device can be decreased to increase light emissionefficiency. In addition, the light emission efficiency can further beincreased with a composition modulation effect by In included in theInAlGaN quaternary alloy. It is to be noted that, the nitridesemiconductor substrate has conductivity of a first conductivity type,and can be any nitride semiconductor such as a GaN substrate, anAl_(x)Ga_(1-x)N substrate (0<x<1), or an AlN substrate included in theAl_(x)Ga_(1-x)N substrate.

Another light-emitting device according to the present invention has anAl_(x1)Ga_(1-x1)N layer (0≦x≦1) of a first conductivity type, anAl_(x2)Ga_(1-x2)N layer (0≦x2≦1) of a second conductivity type locatedabove the Al_(x1)Ga_(1-x1)N layer of the first conductivity type, and alight-emitting layer located between the Al_(x1)Ga_(1-x1)N layer of thefirst conductivity type and the Al_(x2)Ga_(1-x2)N layer of the secondconductivity type and including an InAlGaN quaternary alloy, andincludes a nitride semiconductor layer having thickness of at most 100μm in a more distant position from the light-emitting layer than that ofthe Al_(x1)Ga_(1-x1)N layer of the first conductivity type.

The nitride semiconductor layer having thickness of at most 100 μm isformed by etching or abrasion of the aforementioned nitridesemiconductor substrate in the present invention. With thisconstruction, absorption by the nitride semiconductor substrate can beinhibited in addition to decreasing the density of threadingdislocations operating as non-radiative centers and obtaining thecomposition modulation effect by In included in the InAlGaN quaternaryalloy.

A manufacturing method of a light-emitting device according to thepresent invention includes the steps of forming an Al_(x1)Ga_(1-x1)Nlayer (0≦x1≦1) of a first conductivity type on a side of a first mainsurface of a nitride semiconductor substrate, forming a light-emittinglayer including an InAlGaN quaternary alloy on the Al_(x1)Ga_(1-x1a)Nlayer of the first conductivity type, forming an Al_(x2)Ga_(1-x2)N layer(0≦x≦1) of a second conductivity type on the light-emitting layer, andremoving the nitride semiconductor substrate after forming theAl_(x2)Ga_(1-x2)N layer of the second conductivity type.

As GaN, for example, absorbs ultraviolet light having a wavelength of360 nm or shorter, a light output can be increased by the removing orabrasion of the GaN substrate according to the above-described method.As a result, the light output can further be increased. Other nitridesemiconductor may also absorb light of a wavelength region which isdesired to be taken out, and in such situation, the light output can beincreased by removing the nitride semiconductor substrate.

It is to be noted that, “a B layer is located above an A layer” meansthat the B layer is located in a more distant position from the nitridesemiconductor substrate than that of the A layer. The B layer may or maynot contact the A layer.

The foregoing and other objects, features, aspects and advantages of thepresent invention will become more apparent from the following detaileddescription of the present invention when taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an ultraviolet LED according to a first embodiment of thepresent invention.

FIG. 2 shows an emission spectrum of the ultraviolet LED shown in FIG.1.

FIG. 3 shows a relation between an applied current and a light output ofthe ultraviolet LED shown in FIG. 1.

FIG. 4 shows an ultraviolet LED according to a second embodiment of thepresent invention.

FIG. 5 is an enlarged view of a light-emitting layer shown in FIG. 4.

FIG. 6 shows a relation between an applied current and a light output ofthe ultraviolet LED shown in FIG. 4.

FIG. 7 shows an emission spectrum of the ultraviolet LED shown in FIG.4.

FIG. 8 shows respective relations between applied currents and lightoutputs of an ultraviolet LED of an example of the present inventionaccording to a third embodiment of the present invention, and anultraviolet LED of a comparative example.

FIG. 9 shows a stacked structure of an ultraviolet LED according to afourth embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMNTS

Embodiments of the present invention will now be described.

First Embodiment

FIG. 1 shows an LED according to a first embodiment of the presentinvention. In FIG. 1, a stacked structure (an n type GaN layer 2/an ntype Al_(x)Ga_(1-x)N layer 3/an InAlGaN light-emitting layer 4/a p typeAl_(x)Ga_(1-x)N layer 5/a p type GaN layer 6) is formed on a GaNsubstrate 1. An n electrode 11 is arranged on a second main plane, thatis, a backside of GaN substrate 1, while a p electrode 12 is arranged onp type GaN layer 6. Ultraviolet light is emitted from the InAlGaNlight-emitting layer by applying a current to the pair of n electrode 11and p electrode 12. The InAlGaN light-emitting layer has a compositionof In_(xa)Al_(ya)Ga_(1-xa-ya)N.

The GaN-based I-ED shown in FIG. 1 is manufactured by the followingprocessing steps. A GaN substrate having thickness of 400 μm, adislocation density of 5E6 cm⁻² and a resistivity of 1E-2 Ωcm wasarranged on a susceptor within an MOCVD (Metal Organic Chemical VaporDeposition) device. While the inside of the deposition reactor was keptdecompressed, a stacked structure was formed by an MOCVD methoddescribed below to manufacture an ultraviolet light-emitting diode.

As materials for the MOCVD, trimethylgallium, trimethylaluminum,trimethylindium adduct, ammonia, tetraethylsilane, andbis(ethylcyclopentadienyl)magnesium were used. First, n type GaN layer 2having thickness of 0.1 μm was formed as a ground layer on GaN substrate1 at a growth temperature of 1050° C., and then n typeAl_(0.18)Ga_(0.82)N layer 3 having thickness of 0.2 μm was formedthereon.

Thereafter, the growth temperature was decreased to 830° C., and InAlGaNlight-emitting layer 4 of 60 nm was grown. Flow rates of material gasesin this step were as follows: 2 l/min for ammonia, 3 μmol/min fortrimethylgallium, 0.5 μmol/min for trimethylaluminum, and 60 μmol/minfor trimethylindium adduct. Then, the growth temperature was increasedagain to 1050° C., and p type Al_(0.18)Ga_(0.82)N layer 5 havingthickness of 0.2 μm was formed. Furthermore, the p type GaN layer havingthickness of 30 nm was grown thereon as a contact layer.

To an LED epitaxial structure grown as above, semitransparent pelectrode 12 was formed on p type GaN layer 6 and n electrode 11 wasformed on a second main plane of the GaN substrate opposite to theepitaxial layer (backside) with suitable metal materials. Theultraviolet light-emitting diode manufactured as above has a structureas shown in FIG. 1.

When a continuous current was applied to the above-described ultravioletlight-emitting diode, a band-edge emission of the InAlGaN light-emittinglayer at a wavelength of 360 nm was obtained, as shown in FIG. 2. Evenwhen a value of the applied current was increased up to 300 mA, thelight output was increased linearly without being saturated, as shown inFIG. 3. This result has demonstrated a high heat dissipation property ofthe GaN substrate. In addition, as a substrate having a low dislocationdensity was used as the GaN substrate in this embodiment, the threadingdislocation density was decreased and light emission efficiency could beincreased.

Second Embodiment

FIG. 4 shows an ultraviolet light-emitting diode according to a secondembodiment of the present invention. As compared with the stackedstructure of the ultraviolet LED shown in FIG. 1, this ultravioletlight-emitting diode is characterized in that, anIn_(x)Al_(y)Ga_(1-x-y)N layer 17 as a buffer layer is arranged incontact with light-emitting layer 4 on a side nearer to GaN substrate 1.In addition, the light-emitting layer has a multiple quantum wellstructure, which will be described below.

A manufacturing method of the ultraviolet LED according to thisembodiment is as follows. A substrate having thickness of 400 μm and athreading dislocation density of 5E6/cm² was used as GaN substrate 1. Ntype GaN layer 2 and n type Al_(x)Ga_(1-x)N layer 3 were successivelyformed on GaN substrate 1 by a method similar to that in the firstembodiment. Then, In_(x)Al_(y)Ga_(1-x-y)N buffer layer 17 havingthickness of 50 nm was grown at a growth temperature of 830° C. incontact with n type Al_(x)Ga_(1-x)N layer 3.

Thereafter, a two-layered structure (an In_(x5)Al_(y5)Ga_(1-x5-y5)Nbarrier layer 4 a/an In_(x4)Al_(y4)Ga_(1-x4-y4)N well layer 4 b) wasstacked for three cycles on In_(x)Al_(y)Ga_(1-x-y)N buffer layer 17 toform the multiple quantum well structure, as shown in FIG. 5. In thesecond embodiment, this multiple quantum well structure constructslight-emitting layer 4.

Flow rates of material gases for growing the In_(x)Al_(y)Ga_(1-x-y)Nbuffer layer and the In_(x4)Al_(y4)Ga_(1-x4-y4)N barrier layer were asfollows: 2 l/min for ammonia, 1.5 μmol/min for trimethylgallium, 0.65μmol/min for trimethylaluminum, and 30 μmol/min for trimethylindiumadduct.

Flow rates of material gases for growing the In_(x4)Al_(y4)Ga_(1-x4-y4)Nwell layer were as follows: 2 l/min for ammonia, 1.5 μmol/min fortrimethylgallium, 0.52 μmol/min for trimethylaluminum, and 53 μmol/minfor trimethylindium adduct.

This embodiment is different from the first embodiment in two points.That is, the In_(x)Al_(y)Ga_(1-x-y)N layer as a buffer layer wasarranged, and the light-emitting layer was made to have the multiplequantum well structure of InAlGaN layers.

With the above-described two improvements, a light emission output hasmarkedly increased, as shown in FIG. 6. While the light output for theapplied current of 100 mA was approximately 0.01 mW in FIG. 3, forexample, the light output for the applied current of 100 mA in FIG. 6was largely increased to 1.7 mW, that is, more than 150 times. Inaddition, a half-width of the emission spectrum was decreased to 12 nm,as shown in FIG. 7. This is because, as the light-emitting layer has themultiple quantum well structure, the light emission between quantumlevels becomes dominant.

Third Embodiment

In a third embodiment of the present invention, light outputs werecompared between an ultraviolet LED formed on the GaN substrate (anexample of the present invention) and an ultraviolet LED formed on a GaNtemplate (a substrate formed by growing an n type GaN for 3 μm on asapphire substrate via a GaN buffer layer grown at low temperature) (ancomparative example). The GaN template used was made previously. Both ofthe aforementioned example of the present invention and comparativeexample were formed to have stacked structures as shown in FIGS. 4 and 5except that, as a backside of the GaN template is an insulator, the nelectrode for the GaN template was formed on the previously exposed ntype GaN layer.

For manufacturing, both GaN substrate and GaN template were arrangedtogether on the susceptor within the MOCVD device. Then, the n type GaNlayer, n type Al_(x1)Ga_(1-x1)N layer and In_(x)Al_(y)Ga_(1-x-y)N layeras a buffer layer were formed on each of the GaN substrate and GaNtemplate. Thereafter, a two-layered structure (anIn_(x4)Al_(y4)Ga_(1-x4-y4)N barrier layer/an In_(x3)Al_(y3)Ga_(1-x3-y3)Nwell layer) was stacked for three cycles to form the multiple quantumwell structure as in the second embodiment. Thereafter, a p typeAl_(x2)Ga_(1-x2)N layer/a p type GaN layer were formed, and the pelectrode and n electrode were formed. During the above-describedformation process, growth temperatures and flow rates of material gaseswere the same as those in the second embodiment. As described above, then electrode for the GaN template was formed on the n type GaN layer.

Currents were applied to both of the example of the present inventionand the comparative example manufactured as above, and the light outputswere measured. Results are shown in FIG. 8 in a comparative form. InFIG. 8, values corresponding to five times those of the actual lightoutputs are indicated for the comparative example using the GaNtemplate.

According to FIG. 8, an output obtained with the LED on the GaNsubstrate with a current of 50 mA is about ten times that with the LEDon the GaN template. In addition, though the output of the LED using theGaN template tends to be saturated with a current of 100 mA, the outputof the LED on the GaN substrate increases linearly. Therefore, the GaNsubstrate having a low dislocation density is effective in increasingefficiency of the ultraviolet LED using the InAlGaN light-emitting layerand increasing the output of LED by application of a large current. Highpower as described above with the LED of the example of the presentinvention could be obtained because an increase in temperature by heatemission was suppressed by using the GaN substrate having good heatconduction property, and non-radiative centers were decreased because ofthe low threading dislocation density.

Fourth Embodiment

FIG. 9 shows a stacked structure of a light-emitting device according toa fourth embodiment of the present invention. First, a manufacturingmethod thereof will be described. An Al_(x)Ga_(1-x)N substrate (x=0.18)was arranged on a susceptor, and a stacked structure was manufacturedwhile the inside of a metal organic chemical vapor deposition reactorwas kept decompressed to obtain an ultraviolet light-emitting diodestructure. Trimethylgallium, trimethylaluminum, trimethylindium adduct,ammonia, tetraethylsilane, and bis(ethylcyclopentadienyl)magnesium wereused as materials. First, an n type Al_(0.18)Ga_(0.82)N buffer layer 22having thickness of 0.5 μm was grown at a growth temperature of 1050° C.

Thereafter, the growth temperature was decreased to 830° C. and alight-emitting layer 24 having three cycles of an InAlGaN barrier layer24 a and an InAlGaN well layer 24 b was formed as the second embodimentdescribed above. The growth temperature was then increased again to1050° C. to grow a p type Al_(0.30)Ga_(0.70)N layer 25 having thicknessof 20 nm and a p type Al_(0.18)Ga_(0.82)N layer 26 having thickness of50 nm.

On p type AlGaN layer 26 of the LED epitaxial structure formed as above,semitransparent p electrode 12 was formed with a metal material, while nelectrode 11 was formed on a backside of an AlGaN substrate 21.

When a continuous current was applied to the ultraviolet light-emittingdiode formed as described above, a band-edge emission of the InAlGaN ata wavelength of 351 nm could be obtained. When the applied current was100 mA, an 8 mW light output of the band-edge emission could beobtained.

Additional descriptions of embodiments of the present inventionincluding the above-described embodiments are itemized in the following.

The aforementioned nitride semiconductor substrate can be a GaNsubstrate. Since a large and inexpensive GaN substrate is available, itis suitable for mass production. A threading dislocation density of theGaN substrate is preferably at most 1E7 cm⁻². With this, a threadingdislocation density in the light-emitting device of the presentinvention can be decreased, and thus a density of non-radiative centerscan be decreased.

In addition, the aforementioned nitride semiconductor substrate can bean Al_(x)Ga_(1-x)N substrate (0<x≦1). Crystallinity of the InAlGaNlight-emitting layer can be enhanced by using the Al_(x)Ga_(1-x)Nsubstrate. That is, a difference of lattice constant between thelight-emitting layer and the nitride semiconductor substrate can bedecreased so that a lattice mismatch generated in the light-emittinglayer can be suppressed.

A threading dislocation density of the Al_(x)Ga_(1-x)N substrate (0<x≦1)is preferably at most 1E7 cm⁻². With this construction, a threadingdislocation density in the light-emitting device of the presentinvention can be decreased, and thus a density of non-radiative centerscan be decreased.

Band gap energy of the Al_(x)Ga_(1-x)N substrate (0<x≦1) can be made notmore than energy corresponding to a wavelength of light emitted by thelight-emitting layer including the InAlGaN quaternary alloy. With suchband gap of the nitride semiconductor substrate, light emitted from thelight-emitting layer is not absorbed by the nitride semiconductorsubstrate and can be utilized efficiently.

The construction may include an Al_(x1)Ga_(1-x1)N layer (0≦x1≦1) of afirst conductivity type on a side of a first main surface of the nitridesemiconductor substrate, an Al_(x2)Ga_(1-x2)N layer (0≦x2≦1) of a secondconductivity type located in a more distant position from the nitridesemiconductor substrate than that of the Al_(x1)Ga_(1-x1)N layer of thefirst conductivity type, and the InAlGaN quaternary alloy between theAl_(x1)Ga_(1-x1)N layer of the first conductivity type and theAl_(x2)Ga_(1-x2)N layer of the second conductivity type.

With the construction as described above, highly effective lightemission can be attained by passing a current from the p conductivitytype layer and the n conductivity type layer to the InAlGaN quaternaryalloy sandwiched therebetween.

The construction may include a nitride semiconductor layer of the samekind as the nitride semiconductor substrate of the first conductivitytype between the nitride semiconductor substrate and theAl_(x1)Ga_(1-x1)N layer of the first conductivity type.

With this construction, crystallinity of the Al_(x1)Ga_(1-x1)N layer ofthe first conductivity type can be enhanced as compared with thestructure including the Al_(x1)Ga_(1-x1)N layer of the firstconductivity type formed in contact with the nitride semiconductorsubstrate by allowing the nitride semiconductor layer of the same kindas the nitride semiconductor substrate of the first conductivity type tofunction as a buffer layer.

The construction may include an Al_(x3)Ga_(1-x3)N layer (0≦x3<1, x3<x2)of the second conductivity type having thickness of 1-500 nm on theAl_(x2)Ga_(1-x2)N layer of the second conductivity type.

With the construction as described above, a contact resistance can bemade lower as compared with the construction including the electrodeformed in contact with the Al_(x2)Ga_(1-x2)N layer of the secondconductivity type, which can increase power-light conversion efficiency.The Al_(x3)Ga_(1-x3)N layer of the second conductivity type havingthickness smaller than 1 nm cannot provide a layer sufficient todecrease the contact resistance. In addition, an amount of absorption ofultraviolet light increases in the Al_(x3)Ga_(1-x3)N layer havingthickness larger than 500 nm. Therefore, the Al_(x3)Ga_(1-x3)N layer ofthe second conductivity type should have the thickness within a range1-500 nm.

The construction can include a first electrode formed on a second mainsurface opposite to the first main surface, and a second electrodepaired with the first electrode formed on the Al_(x2)Ga_(1-x2)N layer ofthe second conductivity type.

With the construction as described above, as the first electrode can bearranged on the second main surface, that is, a backside of the nitridesemiconductor substrate, a series resistance can be made smaller. As aresult, voltage efficiency is increased and heat emission can belowered, which can increase the light emission efficiency. Furthermore,the nitride semiconductor has a good thermal conductivity and is lesssusceptible to heat emission, which is also advantageous.

Total thickness of the Al_(x1)Ga_(1-x1)N layer (0≦x1≦1) of the firstconductivity type and the Al_(x2)Ga_(1-x2)N layer (0≦x2≦1) of the secondconductivity type may be at most 0.4 μm.

The total thickness of 0.4 μm or smaller is preferable because a crackwill be generated when the total thickness of the Al_(x1)Ga_(1-x1)Nlayer of the first conductivity type and the Al_(x2)Ga_(1-x2)N layer ofthe second conductivity type becomes larger than 0.4 μm, and light willbe emitted from only a portion of these layers.

The light-emitting device as described above can emit light within awavelength range of 330-370 nm by light emission of the light-emittinglayer.

By adjusting the light-emitting layer so as to radiate the wavelengthwithin the aforementioned range, a light-emitting device of anultraviolet region having high light emission efficiency can beattained.

The aforementioned light-emitting layer may have a construction having aquantum well structure including a well layer indicated asIn_(x4)Al_(y4)Ga_(1-x4-y4)N (0<x4<0.2, 0<y4<0.5) and a barrier layerindicated as In_(x5)Al_(y5)Ga_(1-x5-y5)N (0≦x5<0.2, 0<y5<0.5).

The light emission efficiency can be increased substantially by makingthe light-emitting layer to have the quantum well structure. Inaddition, distortion can be decreased by using InAlGaN crystals for boththe well and barrier layers, resulting in increase in the light emissionefficiency.

The construction may include an In_(x)Al_(y)Ga_(1-x-y)N layer (0<x<0.2,0<y<0.5) having thickness of 10-200 nm between the light-emitting layerand the nitride semiconductor substrate.

With the construction as described above, strain of the light-emittinglayer can be decreased, which can suppress spatial separation ofelectrons and holes by piezo effect and increase the light emissionefficiency.

In another light-emitting device of the present invention, by etching orabrasion of the nitride semiconductor substrate, a nitride semiconductorlayer in a more distant position from the light-emitting layer than thatof the Al_(x1)Ga_(1-x1)N layer of the first conductivity type may beabsent.

With this construction, absorption of a short wavelength region by thenitride semiconductor substrate (nitride semiconductor layer) can beeliminated.

Although the present invention has been described and illustrated indetail, it is clearly understood that the same is by way of illustrationand example only and is not to be taken by way of limitation, the spiritand scope of the present invention being limited only by the terms ofthe appended claims.

1. A light-emitting device, comprising: a nitride semiconductorsubstrate having a dislocation density of at most 1E7 cm⁻²; and alight-emitting layer including an InAlGaN quaternary alloy on a side ofa first main surface of said nitride semiconductor substrate, whereinsaid light-emitting layer has a quantum well structure including a welllayer and a barrier layer; and said InAlGaN quaternary alloy forms saidwell layer.
 2. The light-emitting device according to claim 1, whereinsaid nitride semiconductor substrate is a GaN substrate.
 3. Thelight-emitting device according to claim 2, wherein said dislocationdensity is a threading dislocation density of said nitride semiconductorsubstrate.
 4. The light-emitting device according to claim 1, furthercomprising: an Al_(x1)Ga_(1-x1)N layer (0≦x1≦1) of a first conductivitytype on the side of the first main surface of said nitride semiconductorsubstrate; and an Al_(x2)Ga_(1-x2)N layer (0≦x2≦1) of a secondconductivity type located in a more distant position from said nitridesemiconductor substrate than a position of said Al_(x1)Ga_(1-x1)N layerof the first conductivity type relative to said nitride semiconductorsubstrate; wherein said InAlGaN quaternary alloy of said light-emittinglayer is located between said Al_(x1)Ga_(1-x1)N layer of the firstconductivity type and said Al_(x2)Ga_(1-x2)N layer of the secondconductivity type.
 5. The light-emitting device according to claim 4,further comprising a nitride semiconductor layer of the firstconductivity type formed of a material of which said nitridesemiconductor substrate is formed, between said nitride semiconductorsubstrate and said Al_(x1)Ga_(1-x1)N layer of the first conductivitytype.
 6. The light-emitting device according to claim 4, furthercomprising: a first electrode formed on a second main surface of saidnitride semiconductor substrate opposite to said first main surface; anda second electrode paired with said first electrode and formed on saidAl_(x2)Ga_(1-x2)N layer of the second conductivity type.
 7. Thelight-emitting device according to claim 4, wherein a total thickness ofsaid Al_(x1)Ga_(1-x1)N layer (0≦x1≦1) of the first conductivity type andsaid Al_(x2)Ga_(1-x2)N layer (0≦x2≦1) of the second conductivity type isat most 0.4 μm.
 8. The light-emitting device according to claim 1,wherein said light-emitting layer is adapted to emit light within awavelength range of 330 nm to 370 nm.
 9. The light-emitting deviceaccording to claim 1, wherein said quantum well structure has said welllayer indicated as In_(x4)Al_(y4)Ga_(1-x4-y4)N (0<x4<0.2, 0<y4<0.5) andsaid barrier layer indicated as In_(x5)Al_(y5)Ga_(1-x5-y5)N (0≦x5<0.2,0<y5<0.5).
 10. The light-emitting device according to claim 1, furthercomprising an In_(x)Al_(y)Ga_(1-x-y)N (0<x<0.2, 0<y<0.5) layer having athickness of 10 nm to 200 nm between said light-emitting layer and saidnitride semiconductor substrate.